The digestive and respiratory systems

Cells need oxygen to get energy from food. The body stores little oxygen, so we breathe all the time. The brain controls breathing by itself, so we breathe even in sleep. Air enters through the nose, where it is cleaned of dust, moistened and warmed. That is why it is better to breathe through the nose. Then air goes down the trachea (windpipe) and through two bronchi to the lungs. The lungs are spongy and full of blood vessels. Here blood hands over carbon dioxide and takes up oxygen. The diaphragm, a muscle below the lungs, helps breathing: when it moves down, the lungs expand and fill with air; when it relaxes, air is pushed out. Red blood cells carry oxygen to the whole body. To keep the air clean, air rooms often, stay away from smoke and dust, and do sports.